The localization and adsorption of benzene and propylene in ITQ-1zeolite: grand canonical Monte Carlo simulations

T.J. Hou, L.L. Zhu, Y.Y. Li, X.J. Xu*

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People's Republic of China

Received 27 March 2000; accepted 6 April 2000

Abstract

Grand canonical Monte Carlo (GCMC) simulations have been performed to investigate the localization and adsorption

behavior of benzene and propylene, which are involved in the cumene synthesis process, in purely siliceous MWW zeolite

(ITQ-1). From the mass clouds of GCMC simulations, it can be seen that the benzene and propylene molecules show different

localization and adsorption behavior in the zeolite cavities. In the 10-MR channels, both benzene and propylene show high

localization. In the 12-MR supercages, the propylene molecules cannot only almost ®ll all the possible positions in one

supercage, but also can be steadily located in the short 10-MR conducts interconnecting the 12-MR supercages, where the

benzene molecules are adsorbed close to three adsorption sites. By analyzing the location of benzene and propylene in ITQ-1, it

can be deduced that the alkylation of benzene and propylene will happen mainly in 12-MR supercages. Moreover, a series of

simulations have been performed to predict the adsorption isotherms of benzene and propylene at 315 K and 0±1.4 kPa. The

results for benzene generally are in agreement with the trend from experiments on a series of aromatic compounds. The results

reveal that at low pressures, the loading of propylene is lower than that of benzene, which seems to be caused by the relatively

unfavorable potential interactions between propylene/zeolite and propylene/propylene. q 2001 Elsevier Science B.V. All

rights reserved.

Keywords: ITQ-1; Grand canonical Monte Carlo simulations; Adsorption; Benzene; Propylene

1. Introduction

The alkylation of benzene with propylene to

produce cumene, a starting material for the production

of acetone and phenol [1], is very important in hydro-

carbon processes. Traditionally, some mineral acids

including AlCl3 and others are employed, which

present good catalytic performance [2,3], while rais-

ing serious environmental problems such as corrosion

and waste disposal. In order to overcome some draw-

backs of the traditional mineral acid catalysts, tech-

nologies such as the Mobil-Badger process using H-

ZSM-5 [4], the CDTEC and ENI process using Y and

b zeolites [5] and the DOW process using Mordenite

[6] have made great progress in recent years. The

zeolites are considered to be cleaner catalysts; more-

over, they can effectively reduce the amount of less

desired products such as diisopropylbenzenes, but

introduce the formation of n-propylbenzene which is

not formed in signi®cant amounts when using mineral

acid catalysts [7].

It is well known that in the case of zeolites, the pore

dimensions and the diffusion of the reactants and

products signi®cantly contribute to reactions that

occurred in zeolites, since the reactivity and ®nal

Journal of Molecular Structure (Theochem) 535 (2001) 9±23

0166-1280/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.

PII: S0166-1280(00)00520-0

www.elsevier.nl/locate/theochem

* Corresponding author.

E-mail address: xiaojxu@chemms.chem.pku.edu.cn (X.J. Xu).

product distributions will depend highly on the zeolite

channel structure and the diffusion of the sorbate

molecules. For instance, the formation of n-propylto-

luene has been observed with ZSM-5 but not with

Mordenite, which suggests that the transalkylation

reaction occurs in the 10-MR channel intersection of

ZSM-5, whereas the intersection of 12-MR and 8-MR

channels in Mordenite does not provide suf®cient

space for the bimolecular transalkylation process to

occur. However, the yield of transalkylation in zeolite

Y is found to be much lower than that in ZSM-5 under

the same reaction conditions, which suggests that the

reactivity cannot be determined only by the size of

cavities in which the reactions take place, but the

diffusion of the chemical compounds involved in the

reaction also needs to be taken into account. With

respect to energy, the energetic complementarity

between the zeolite and the chemical compounds

involved in the reaction is simply expressed as

shape adsorption or shape selectivity of the zeolite.

The diffusion of the adsorbents in the zeolite is really

a ªdockingº process, and they always prefer to follow

an energetically favorable pathway.

Several zeolites including H-ZSM-5, USY, b and

MCM-22 have been tested for the alkylation of

benzene with propylene, and it has been found that

MCM-22 is very reactive [8]. Zeolite MCM-22 (IZA

code MWW) is a novel zeolite discovered recently by

scientists at Mobil [9,10]. Compared with other

common types of zeolites, MCM-22 possesses an

interesting and unusual framework structure: two

independent pore systems formed by interconnected

sinusoidal 10-MR pores with a 4±5.5 AÊ diameter and

an independent 12-MR supercage with 18.2±7.1 AÊ

linked by 10-MR windows. The unusual framework

topology, high thermal stability, large surface area

and good adsorption capacity render this zeolite

very interesting for catalysis. Several previous studies

have looked into the diffusivities of benzene and

cumene in the MWW structure [11,12]. It has been

found that the diffusion and adsorption behaviors of

these molecules are different in the two independent

pore systems. The simulations of Perego et al. [11]

indicated that the diffusion of cumene was more

favorable in the 12-MR supercages than in the 10-

MR sinusoidal system, although the 10-MR windows

interconnecting the 12-MR supercages still presented

a diffusion barrier for cumene. The molecular

dynamics simulations of Sastre et al. [12] revealed

that benzene was not seen to diffuse through two

pore systems, and only intracage mobility was seen

in the supercage voids where a certain activation

energy must be necessary for a benzene molecule

migrating between 12-MR supercages through 10-

MR windows.

Until now, few studies have been performed to

investigate the localization and adsorption of benzene

and propylene in MCM-22, and the favorable adsorp-

tion sites of those molecules in zeolite lattices are not

clear. As far as we know, no work has been under-

taken yet to investigate the adsorption properties of

zeolite and propylene in MWW type zeolites through

grand canonical Monte Carlo simulations (GCMC)

simulations. In the present work, the GCMC simula-

tion technique was used to predict the adsorption char-

acteristics of benzene and propylene in the ITQ-1

zeolite. We intended to determine the potential

adsorption sites of benzene and propylene in ITQ-1

lattice, as well as the properties of the localization of

benzene and propylene in the zeolite. In the mean-

time, we wanted to predict the adsorption isotherms

for these two kinds of molecules.

2. Method

2.1. Model representations and potential force®eld

Considering the high Si/Al ratio of the MCM-22

type zeolite and the dif®culty of determining Al distri-

butions in disordered zeolites by experiments, mean-

while, in order to simplify the simulations, the pure

siliceous analogue of MCM-22, ITQ-1, was adopted

in this paper. The model of the ITQ-1 was constructed

according to the results from Camblor et al. [13]. In

the simulations, the silicon and oxygen atoms of the

zeolite framework were assumed to be ®xed at their

crystallographic positions from X-ray diffraction

studies. The benzene and propylene molecules were

rigid, that is to say, they could only translate and

rotate, but were not allowed to deform.

The zeolite and the sorbates were assumed to

interact via a pairwise-additive potential between

atoms of the guest molecules and atoms of the

zeolite. The site±site interactions are models with a

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±2310

Lennard-Jones plus Coulomb potential,

V�rij� � Dij

�R0�ijRij

" #12

22�R0�ij

Rij

" #6( )1

qiqj

Rij

�1�

where i and j indicate atoms of the sorbate and of the

zeolite, respectively, and Rij is the distance between

them. Dij and �R0�ij are the Lennard-Jones parameters,

and qi and qj are the partial charge of the atoms. Six

different atom types were considered in the studied

system, including O_z (O atom in zeolite framework),

Si (Si atom in zeolite framework), C_R (C atom in

benzene ring), C_3 (sp3 C in propylene), C_2 (sp2 C in

propylene) and H (H atom in benzene and propylene).

The Lennard-Jones parameters for O_z and Si origin-

ally derived by Burchart [14], and those for other atom

types taken from Mayo [15], are listed in Table 1.

Then, the off-diagonal van der Waals parameters for

each pair of atoms were calculated based on the

geometric mean. The partial charges for O_z

(20.19ueu) and Si (10.38ueu) were taken from the

calculations of Burchart. The above hybrid force®eld

has been tested by MSI and distributed as a Burchart±

Dreiding force ®eld [16]. The partial charges for the

atoms in benzene and propylene (Fig. 1) were

computed using the AM1 method, available in

MOPAC 7.0 [17].

2.2. Grand canonical Monte Carlo simulations

The GCMC simulation may be the most common

technique for predicting the zeolite adsorption phase

equilibria from molecular simulations [18±20]. The

GCMC simulation technique simulates the equili-

brium of a collection of adsorbates in a micropore at

constant chemical potential, volume, and temperature

or pressure. In the GCMC simulations, the number of

particles in the system is not ®xed, but the chemical

potential of each species is ®xed. Sorption translates

the chemical potential into the partial pressure (or

fugacity) of each component. Equilibrium is achieved

when the temperature (Tframe) and the chemical poten-

tial (m frame) of the gas inside the framework are equal

to the temperature (Tgas) and chemical potential (m gas)

of the free gas outside the framework. For a non-ideal

gas, the chemical potential depends upon fugacity ( f ),

which is a function of both temperature and pressure.

The bulk pressure can be determined from the chemi-

cal potential using a Lennard-Jones equation. So the

GCMC simulation technique enables one to study

many important characteristics of zeolite systems

under certain pressure and temperature.

Eight unit cells of zeolite with a total of 1728 atoms

were used to construct the simulation box, and peri-

odic boundary conditions were applied in three

dimensions in order to simulate an in®nite (macro-

scopic) system. In order to achieve the real equilibra-

tion of the system, the length of the simulations was

totally 6 £ 106 steps, and every 600 steps, a con®gura-

tion of the system was recorded. The ®rst three million

steps were used for equilibration and not included in

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±23 11

Table 1

Lennard-Jones parameter for six types of atoms

Atom type D0 (kcal mol21) R0 (AÊ )

O_z 0.1648 3.3000

Si_z 0.0496 4.2000

C_R 0.0951 3.8983

C_3 0.0951 3.8983

C_2 0.0951 3.8983

H 0.0152 3.1950

Fig. 1. Partial charges for: (a) benzene; and (b) propylene used in calculating the zeolite/sorbates and sorbates/sorbates interaction energy, given

in units of the electron charges ueu.

T.J.

Ho

uet

al.

/Jo

urn

al

of

Mo

lecula

rStru

cture

(Theo

chem

)535

(2001)

9±

23

12

Fig. 2. Skeletal drawings of the framework structure of ITQ-1 (MWW structure type). (a) Schematic view of the independent pore systems along the yz axis. (b) Schematic view of

the independent pore systems along the xy axis. (c) Two adjacent 12-MR supercages. The sinusoidal 10-MR channels are all interconnected to each other, and multiple diffusion

trajectories can be allowed to every diffusing molecule. The 12-MR are independent with 10-MR channels, which are interconnected through short 10-MR windows.

the averaging. A cutoff of 10 AÊ was applied to the

Lennard-Jones interactions, and the long-range elec-

trostatic interactions were calculated by using the

Ewald summation technique. The Ewald summation

to calculate the adsorbate±adsorbent and adsorbate±

adsorbate is generally time-consuming. We therefore

used a grid-interpolation procedure in which the simu-

lation boxes were split into a collection of small

cubes. The grid-interpolation method allows us to

take into account any degree of accuracy in the

description of the adsorbate/zeolite and adsorbate/

adsorbate interaction energy since all the needed

grids are calculated separately prior to any simulation

runs. First, the GCMC simulations were carried out in

the condition of 300 K and 1 atm. Then, a series of

simulations were performed to predict the adsorption

isotherms for benzene and propylene at 315 K. All

calculations were performed in the Cerius2 molecular

simulation package [16] on a SGI Octane 2-CPU

workstation.

In this paper, we also computed the potential

energy of the diffusing benzene molecules along

their trajectory, which allowed estimation of the

energy pro®les of one benzene molecule while it

diffused between two adjacent 12-MR supercages

through 10-MR windows in the ITQ-1 zeolite. Conse-

quently, the activation energy of one benzene mole-

cule through 10-MR windows was predicted.

3. Results and discussion

A deep insight into the channel systems present in

ITQ-1 (Fig. 2) reveals some special features that will

greatly affect the location and adsorption of sorbates

in the zeolite lattice. First, all 10-MR sinusoidal chan-

nels are interconnected with each other and have high

tortuosity. The benzene molecules and even the

propylene ones may be restricted through sinusoidal

channels in ITQ-1. Second, the larger 12-MR cavities

have large dimensions with 7.1 AÊ £ 18.2 AÊ , so they

are expected to host both benzene and propylene

whose mobility and location inside the supercages

will be very interesting to investigate. Additionally,

we are also interested in knowing whether they

migrate from one cavity to another nearby through

10-MR openings or tend to remain inside a given

cavity. Certainly, all these features will depend on

the conditions of the environment including tempera-

ture and pressure.

3.1. The in¯uence of the minor distortions of

framework to simulation results

In the calculations, the zeolite atoms were ®xed at

their crystallographic positions. Generally, the frame-

work of zeolite is relatively rigid and possesses good

thermal stability. For instance, the introduction of

xylene into the cavities of the faujasite NaY has little

effect on the framework structure [21]. In order to

examine the in¯uence of the minor change of zeolite

lattice on the simulation results, the framework of the

ITQ-1 zeolite was manually adjusted to obtain three

transformative structures: the ®rst structure was

obtained by expanding (by averages) the coordinates

of all atoms by 1% along the a and b directions; the

second one was obtained by expanding (by averages)

the coordinates by 1.5% along the a and b directions;

and the third one was obtained by expanding (by

averages) the coordinates by 2% along the c direction.

Compared with the volume of the crystallographic

structure, the volumes of these three adjusted

structures are 1, 2.75 and 2% larger (the cell para-

meters for these three adjusted models are listed in

Table 2). For these three adjusted structures and the

crystallographic structure, the simulations of benzene

in ITQ-1 were performed by using GCMC simulations

at standard temperature and pressure.

The full loadings of benzene and the interaction

energy for most probable distributions (Table 2) indi-

cate that these four models do not show noticeable

differences. Moreover, four mass clouds for benzene

molecules are similar, indicating that the minor

distortion of the zeolite framework does not have

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±23 13

Table 2

The cell parameters and the GCMC simulation results for four

models

Model a (AÊ ) b (AÊ ) c (AÊ ) Loading E (kJ mol21)a

1 14.2081 14.2081 24.9452 59.71 281.9

2 14.3502 14.3502 24.9452 58.14 281.1

3 14.4212 14.4212 24.9452 62.55 282.3

4 14.2081 14.2081 25.4410 60.11 280.7

a The average interaction energy between benzene and the zeolite

framework.

considerable effect on the simulation results. So in this

paper, the crystallographic structure of ITQ-1 is

adopted and the zeolite ¯exibility is ignored.

3.2. The diffusions of benzene in the two independent

channel systems

The evolution of the loading and energy of benzene

shows that after 6 £ 106 simulation steps, the equili-

bration has been achieved. At 300 K and 1 atm, the

full loading of benzene is about 63 molecules per 8

ITQ-1 unit cells. Fig. 3(a) depicts the energy distribu-

tion of interaction between benzene and ITQ-1

zeolite at full loadings. The energy distribution

is roughly single-peaked, with a maximum

around 219.9 kJ mol21, and a shoulder from

217.5±215.0 kJ mol21.

In order to characterize the location of the adsorbed

molecules in the ITQ-1 zeolite, several mass clouds

were depicted. As a powerful analysis tool, the mass

cloud shows the preferred positions of the sorbates in

the zeolite. The mass cloud of benzene, with respect to

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±2314

Fig. 3. (a) Benzene/zeolite potential energy distribution. (b) Propylene/benzene potential energy distribution.

the zeolite famework, is shown in Fig. 4. The center of

mass for each sorbate molecule in each con®guration

is displayed as a dot in the model space. From Fig.

4(a), it can be noted that the spatial distribution of

benzene is roughly territorial, which can be divided

into four regions: one in the 10-MR channels and

three others in the 12-MR supercages, which have

been named as S1, S2, S3 and S4. The S1 site is

located in the 10-MR channels. The S2 site is

observed near the 10-MR facing the 6-MR in

supercage, while both the S3 and S4 sites are close

to the central part of the 12-MR supercages. The

distribution of benzene in the present simulations is

somewhat similar to the trajectories of benzene

derived from the previous molecule dynamics simula-

tion performed by Sastre et al. [12].

Fig. 4(c) depicts the mass cloud of benzene with

interaction energy ranging from 2100 to

220 kJ mol21 (the benzene molecules with interac-

tion energy lower than the most probable energy). It

can be observed that the benzene molecules with rela-

tively lower interaction energy are almost adsorbed

close to the S1 site. Fig. 4(b) shows the mass cloud

ranging from 2100 to 218 kJ mol21; besides benzene

adsorbed close to the S1 site, some benzene molecules

near the S2 site also fall into this energy interval.

Comparing three mass clouds in Fig. 4, it is obvious

that the interaction energy of benzene at the S3 and S4

sites is mainly higher than 218 kJ mol21.

The interaction energy of benzene adsorbed close

to the S1 site is only about 220 kJ mol21, but these

molecules seem to move within a restricted area in the

10-MR channels. Although there is a high degree

of tortuosity in the circular channels, the circular

10-MR channels in ITQ-1 are so small (only

4.0 AÊ £ 5.5 AÊ ) that it becomes dif®cult for benzene

to diffuse through the 10-MR sinusoidal channels

of ITQ-1. Our results on the S1 site occupancy are

in good agreement with the recent molecular

dynamics simulations of the diffusion of benzene

and propylene in ITQ-1 [12], which indicates that

the benzene molecules move within a restricted

area around the minimum energy position from

the trajectories of molecular dynamics.

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±23 15

Fig. 4. (a) Mass cloud for benzene in the ITQ-1 cavities. (b) Mass cloud for benzene with interaction energy ranging from 2100 to

218 kJ mol21. (c) Mass cloud for benzene with interaction energy ranging from 2100 to 220 kJ mol21.

For the product of the alkylation of benzene

with propylene, it will be more dif®cult for the

larger cumene to migrate through 10-MR channel

systems. The previous results of Perego's study

compared the diffusion of cumene in MCM-22

and ZSM-5, where the activation energies had

been calculated to be 90.0 and 18.6 kcal mol21

in the sinusoidal channels of the MWW structure

and in the channels of ZSM-5, respectively. The

combination of the present work and the previous

simulations suggests that the benzene and the

larger cumene molecules will ®nd it very dif®cult

to penetrate through the 10-MR sinusoidal

channels.

At the S2 site, the benzene molecules also

possess relatively lower interaction energy,

although a little higher than those at the S1 site,

which distribute in a generally localized manner.

At the S2 site, the plane of benzene is observed to

prefer being parallel with benzene near the S1

site. We assumed that the benzene molecules at

the S2 and S3 sites would produce relatively

strong aromatic stacking interactions.

The other two interesting sites are located near

the center of the 12-MR supercages (the S3 and

S4 sites). In our model, the interaction energy of

benzene adsorbed close to those two sites is

higher than 218 kcal mol21. It is apparent from

Fig. 4(a) that the benzene molecules near the S3

and S4 sites are considerably delocalized in the

vicinity of their preferred sites of adsorption,

which are quite different from those near the S1

and S2 sites.

Each supercage is connected to six other super-

cages through 10-MR windows, and therefore,

intercage motion is, in principle, possible. But

partly due to the size and the position of the 10-

MR interconnecting windows, the benzene molecules

near them should be energetically unfavorable,

and relatively high activation energy must be

needed for benzene molecules to migrate from

one supercage to another through 10-MR

windows. In Fig. 4(a), benzene is not observed

in the 10-MR interconnecting region, so it can

be concluded that the migration of benzene mainly

happens in the same supercage, and the intercage

motion is somewhat dif®cult from the viewpoint

of interaction energy. Anyway, the migration of

benzene along 12-MR cavities is much easier

than that along 10-MR channels.

3.3. The diffusions of propylene in the two

independent channel systems

The evolution of the loading and energy of

benzene shows that propylene achieves the equili-

brium faster than benzene. At 300 K and 1 atm,

the full loading of propylene is about 83.7 mole-

cules per 8 ITQ-1 unit cells. The mean total inter-

action energy between the sorbates and the zeolite

is nearly 29.41 £ 102 kcal mol21. Fig. 3(b) depicts

the energy distribution of interaction energy

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±2316

Fig. 5. Mass cloud for benzene in the ITQ-1 cavities along the xy orientation.

between propylene and ITQ-1 zeolite at full

loading. The energy distribution of propylene is

quite similar to that of benzene, roughly single-

peaked and an obvious shoulder. But the interac-

tion energy for propylene is much higher than that

for benzene, which means propylene may be more

unfavorable in zeolite than benzene; moreover, the

shoulder for propylene is not obvious.

The mass cloud of propylene indicates a completely

different picture compared with that of benzene (Fig.

5). From Fig. 6, it can be seen that compared

with benzene, the propylene molecules located in

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±23 17

Fig. 6. (a) Mass cloud for propylene in the ITQ-1 cavities. (b) Mass cloud for benzene with interaction energy ranging from 2100 to

210.5 kJ mol21. (c) Mass cloud for benzene with interaction energy ranging from 2100 to 212 kJ mol21.

Fig. 7. Mass cloud for propylene in the ITQ-1 cavities along the xy orientation.

the 10-MR channels are generally localized, but a

much wider area is covered by the smaller propylene

molecules. Careful inspection of Fig. 6 shows that the

propylene molecules are not observed in the channel

intersections, indicating that the channel intersections

correspond to the locations of higher energy.

Although propylene seems to move more freely in

10-MR channels compared to benzene, a certain acti-

vation energy seems to be needed for propylene to

migrate through the 10-MR channels freely.

A completely different picture can be observed for

propylene (Fig. 7) in the 12-MR supercages compared

with benzene, where the propylene molecules almost

®ll all the possible positions in one supercage. It

should be noted that the propylene molecules can be

steadily located in the short 10-MR conducts around

3 AÊ long, very different from the mass cloud of

benzene. Fig. 6(b) shows the mass cloud of propylene

with interaction energy ranging from 2100 to

212 kJ mol21 (the sorbate molecules with interaction

energy lower than the most probable energy), and it

can be observed that the propylene molecules within

that energetic interval are mainly distributed in the

interconnecting area of 10-MR channels and the

short 10-MR conducts. Obviously, the propylene

molecules located in some areas between two adjacent

12-MR supercages are energetically favorable, which

means that propylene can cross from one 12-MR

supercage to another easily and does not even require

any activation energy.

Considering the localization and adsorption of

propylene and benzene in two separate channel

systems, it can be concluded that the reaction

will mainly occur in supercages, not in 10-MR

channels, because the benzene molecules and the

larger product of alkylation are very dif®cult to

migrate through 10-MR channels. Moreover, it

can be concluded that the alkylation on MCM-22

will be partly diffusion-controlled, and the diffu-

sion of benzene in 12-MR supercages and espe-

cially the out-diffusion of cumene will be the

controlling steps of the alkylation reaction.

3.4. Predictions of adsorption isotherms for benzene

and propylene

In order to investigate the adsorption behavior of

benzene and propylene thoroughly, a series of simula-

tions has been performed to get the adsorption

isotherms. In order to be compared with some experi-

mental results, the temperature of the simulations is

set at 315 K, and the pressure ranges from 0.0 to

3.0 kPa. The calculated adsorption isotherms of pure

propylene and benzene in ITQ-1 at 315 K are shown

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±2318

Fig. 8. Simulated adsorption isotherms of benzene and propylene at 315 K and experimental values for toluene.

in Fig. 8. Because the experimental isotherms for

propylene and benzene are not available, the experi-

mental isotherm of toluene is used for comparison

[22]. Certainly, the isotherms of benzene and toluene

cannot be quantitatively compared, but their struc-

tures are similar and their interactions with the zeolite

lattice are only slightly different, so their adsorption

behavior in the zeolite lattice should be similar to

some extent. Only from the viewpoint of volume,

benzene is relatively small, so its loadings should be

remarkably higher than those of toluene, which can be

well deduced from our simulations (Fig. 8). The

experiments of infrared spectroscopy and adsorp-

tion±microcalorimetric studies have been applied to

the adsorption±diffusion behavior of toluene, meta-

and ortho-xylene, and 1,2,4-trimethylbenzene with

different kinetic diameters in MCM-22 [22], and the

adsorption isotherms have validated that the zeolite

uptake signi®cantly relies on the size of the adsorbate

molecules. It can be observed that the uptake of m-

xylene is about half the value of toluene. The value of

o-xylene is much lower and approximately the same

as that of 1,2,4-trimethylbenzene. From our simula-

tions, it can be noted that the uptake of benzene is

much higher than the value of toluene, which also

accords with previous research [22].

The most obvious difference between the adsorp-

tion isotherms of benzene and propylene is that the

loadings of propylene are signi®cantly larger than

those of benzene at low pressure, which seems not

to agree with the laws derived from the isotherms of

some aromatic compounds. For aromatic compounds,

their structures do not exhibit signi®cant difference,

and interactions between sorbate/zeolite and sorbate/

sorbate should be similar, so the adsorbed amounts are

mainly concerned with the size of the sorbate mole-

cules. In the case of benzene and propylene, their

structures are quite different; besides the geometric

factor, the energetic factor will also contribute a lot

to their loadings. The comparison of benzene and

propylene reveals that the interaction energy between

benzene and the zeolite framework is obviously lower

than that between propylene and the zeolite frame-

work. In previous simulations at 350 K and 1 atm, it

has been validated that the interaction energy for

propylene is much higher than that of benzene,

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±23 19

Fig. 9. Two energy minimum conformers of benzene near the abcd plane in the 10-MR window. The benzene molecule is represented with a

ball-and-stick model.

which means that in zeolite cavities propylene may be

more unfavorable than benzene. So in some condi-

tions, especially under low pressure, the unfavorable

energy will make the uptake of propylene lower than

that of benzene.

3.5. Migration of benzene through the interconnecting

10-MR windows

Previous calculations have hypothesized that the

reaction will mainly happen in 12-MR supercages

and the alkylation process will be partly affected by

diffusion of reactants especially benzene and its

products. Obviously, the intercage motion of benzene

will be the most important factor to be considered. In

order to get a clearer picture about the migration of

benzene from one supercage to another, a single

benzene molecule has been observed following a

very simple path connecting two 12-MR supercages.

During the calculations, the zeolite structure is rigid,

but the conformational ¯exibility of benzene is

allowed to be considered, so the potentials include

three terms: Lennard-Jones and Coulomb potential

between zeolite and benzene, plus the internal poten-

tial of benzene.

Energy minimizations under constraints were used

for the systematic search for local minima. First,

considering that during the migration process, the

potential barrier may be near the 10-MR interconnect-

ing areas, the benzene molecule was placed near the

10-MR window as the starting point of its path. Atoms

a±d (Fig. 9) are four Si atoms in the 10-MR window,

which construct a plane. First, the gravity centers of

the plane and the benzene molecule were superim-

posed. Near the starting point, the benzene molecule

was rotated systematically and the energy minimiza-

tions were performed to ®nd the potential minima.

The results indicate that near this point, two potential

minima for benzene can be found (Fig. 10). Moreover,

from the viewpoint of energy, the ®rst conformer is

preferred. Second, we forced the benzene molecule to

translate through the supercage along the axis of

symmetry, and a simple energy minimization strategy

was applied to optimize the conformation of the

benzene molecule to ®nd the local potential minima.

Third, we divided the corresponding path into small

steps 0.25 AÊ distant from each other, and for all the

corresponding conformations, we calculated the inter-

action energy between the benzene molecules and the

zeolite framework. Fig. 11 shows the minimum

guest±host interaction energy as a function of the

distance between the center-of-mass of benzene and

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±2320

Fig. 10. Energy pro®le for benzene in ITQ-1 as a function of the distance between the center-of-mass of the molecule and the center of the abcd

plane while it is pulled from cage to cage. Points A±C correspond to the potential maximum position, and points D±F correspond to the

potential minimum position.

that of the plane abcd. Because of the symmetry of the

zeolite, the energy function is mirror symmetric with

respect to the plane abcd. Only three crystallographi-

cally different types of energetic minima are

observed.

The minima at 0 AÊ corresponds to the starting

point, while 3.75 and 10.25 AÊ seem to correspond to

the S4 adsorption site inside the supercages. Mean-

while, there exist three potential barriers along the

pathway: A, B, and C (Fig. 11), among which the

potentials at A and C are much higher than that at

B. The energy pro®le of benzene as it is pulled from

cage to cage in the ITQ-1 structure is similar to the

previous simulations by Sastre et al. [12]. Obviously,

the potential barriers should control the diffusion of

benzene through 10-MR windows. From Fig. 11, it

can be seen that A and C points are all near 10-MR

windows, and they are symmetric to plane abcd.

Moreover, the separate contributions to the activation

energy have been calculated (Fig. 10). The ®rst contri-

bution comes from the Lennard-Jones energy, and the

second one comes from the electrostatic interaction

between the benzene molecule and the zeolite frame-

work plus the conformational energy of benzene. As

in Fig. 10, the Lennard-Jones potential obviously

contributes more than the electrostatic potential (the

internal conformational energy of benzene can be

ignored). The energy necessary to cross from cage

to cage is around 24 kcal mol21. Previous calculation

results from molecular dynamics [12] have predicted

T.J. Hou et al. / Journal of Molecular Structure (Theochem) 535 (2001) 9±23 21

Fig. 11. Top view of the ITQ-1 structure showing the potential barriers of benzene if it is pulled from cage to cage. Points A±C correspond to

the potential maximum position.

Fig. 12. Side view of the ITQ-1 structure showing the critical points

(A±F) in the diffusion path followed by the benzene molecule as it

is pulled from cage to cage. Points A±C correspond to the potential

minimum position, and points D±F correspond to the potential

maximum position.

that the values are ranging from 15 to 20 kcal mol21,

which generally agrees with our predictions. The

deviation mainly comes from the different potential

parameters and different partial charges adopted in

this paper.

Along the energy pro®le, there exists another

potential barrier at position B, which is just in the

center of the 12-MR supercage. If we treat the lowest

energy minimum in the benzene path as the adsorption

S4 site, therefore, one benzene molecule that migrates

from one S4 site to another in the same 12-MR superc-

age will only need 6 kcal mol21, which is much lower

than that for intercage migration. That is to say, the

migration of benzene mainly happens in the same

supercage and the intercage motions should need

certain activation energy.

Certainly, the diffusion path of the benzene mole-

cule is very simple and ideal (Fig. 12), and it only

corresponds to very low loadings of benzene. In

normal conditions, especially for high loadings, the

presence of the interaction energy of the sorbates

will in¯uence the activation energy greatly, but the

present study can also afford some useful information

about the diffusion behavior. Because certain large

activation energy is needed for benzene molecules

to cross from one cage to another, increasing the

temperature would possibly increase the probability

of observing benzene intercage motions.

4. Conclusions

GCMC simulations have been performed to simu-

late the location and adsorption of benzene and propy-

lene in a purely siliceous MWW structure (ITQ-1).

Two separate simulation processes have been carried

out to explore the locations and possible adsorption

sites in each channel system of the ITQ-1 at 300 K and

1 atm. From calculation results of different distorted

models, it can be concluded that the minor change of

the zeolite framework does not introduce noticeable

effects on the simulation results. The mass clouds

from GCMC simulations indicate that benzene and

propylene show high localization in 10-MR channels,

which is due to the small size of the 10-MR openings

of the sinusoidal systems. In 12-MR supercages, the

benzene and propylene molecules show quite different

mobilities. The spatial distribution of benzene in a

12-MR supercage can be clearly divided into three

sites of adsorption: one site near the 10-MR facing

the 6-MR in supercage and the other two near the

center of the 12-MR supercages. So we can conclude

that the migration of benzene molecules mainly

happens in the same supercage, and the intercage

motions should need certain activation energy. In

the case of propylene, the sorbate molecules not

only almost ®ll all the possible positions in one

supercage, but also can be steadily located in the

short 10-MR conducts around 3 AÊ long, which

means that propylene can cross from one supercage

to another very easily and does not require obvious

activation energy. The adsorption isotherms of

benzene and propylene at 315 K and 0±3.0 kPa

have been predicted, and the adsorption isotherm

of benzene coincides with the trend from experi-

ments on a series of aromatic compounds. At low

pressure, the loadings of propylene are signi®-

cantly lower than those of benzene, which are

mainly caused by the relatively unfavorable inter-

action energy between propylene and the zeolite

framework.

From the calculations for the activation energy of a

benzene molecule in the intercage process through 10-

MR windows, it can be found that at the simulated

conditions the thermal energy allows the mobility of

benzene inside the cavity but not from one 12-MR

cavity to another. By analyzing the locations of

benzene and propylene in the ITQ-1 framework, the

alkylation of benzene and propylene will mainly

happen in 12-MR supercages at the external surface

or close to the external surface.

Acknowledgements

This project is supported by NCSF 29992590-2 and

29573095.

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